Compositional changes of Coal-Derived Liquids (CDLs) during hydrotreatment on fixed bed catalysts can be investigated by low-voltage (12 eV) mass spectrometry (MS) in combination with multivariate analysis without chromatographic pre-separation. Low-voltage (12 eV) MS data were obtained and compared under two different inlet conditions; (1) MS inlet at ambient temperatures and (2) with the MS inlet preheated to about 200°C. The major trend found in both data sets by Factor analysis was the hydrotreatment effect, namely, hydroaromatic (hydrotreater product) versus aromatic (hydrotreater feed) compounds. A second “time + temperature” trend that reflected the aging of the catalyst as well as the temperature of the hydrotreater reactor was only obtained under ambient inlet temperature conditions. Numerically extracted spectra along the second trend show that more of the low molecular weight components were found early in the process and also at lower reactor temperatures; whereas more of the higher molecular weight components, primarily alkyl substituted polynuclear aromatics, were obtained later in the process and at higher temperatures of the hydrotreater.